The majority of automatic transmissions of motor vehicles available on the market have a parking lock which is mechanically actuated by means of a cable pull connected to a selector lever. New developments provide, in some cases, an electrohydraulic or electromotive actuation means, which is actuated electrically by the selector lever. This is known as full shift by wire function. If a vehicle is equipped with a fully automatic parking function, in which the driver may even be situated outside the vehicle during the parking process, the vehicle must automatically engage the parking lock. This is only possible with a transmission with full shift by wire function.
To avoid new development, or in the case of low equipment rates of the full shift by wire function, and in the case of an approach strongly geared towards using identical parts to form the base transmission, use is made of electromotive parking lock actuators situated outside the transmission. Actuator shift modules are known which are bolted onto the outside of the transmission and which are connected directly to a parking lock shaft of the transmission, and actuator shift modules are known which are fastened to the transmission or to the vehicle and which actuate the parking lock lever via a cable pull.
The vehicle must be capable of being securely parked, on all roadway gradients, even in the event of a failure of the on-board electrical system. Vehicles with an electric parking brake and an electric parking lock actuator therefore require either a second redundant electrical supply or a device which automatically engages the parking lock in this situation.
A parking lock for an automatic transmission, wherein the parking lock is actuable by means of an electric parking lock actuator, wherein the parking lock actuator has an electric motor, a spur gear stage which is driveable by means of the electric motor, and a worm gear stage which is driveable by means of the spur gear stage, wherein a worm shaft of the worm gear stage is connected rotationally conjointly to an output gear of the spur gear stage, and a worm gear of the worm gear stage is connected rotationally conjointly to a transmission-side parking lock shaft, is known from practice. Said electric parking lock actuator is bolted onto the transmission and is used in a purely electrically operated passenger motor vehicle (BMW i3). Said parking lock actuator is not equipped with a fail-safe function, because a redundant electrical supply is already available in electric vehicles.
EP 1 228 328 B1, which is incorporated by reference herein, discloses a transmission actuator arrangement with an electromagnetic clutch which, in the event of an electrical failure, is decoupled only by means of a spring element, and thus the shift position “P” of the transmission and the parking lock is engaged.